Girls' Generation, 2NE1, GD&TOP, JYJ, Wonder Girls and others from South Korea are expanding audiences in U.S. beyond Korean Americans.

The nine young women of Girls' Generation sauntered onto the performance stage of "Late Show With David Letterman." Flanked by a DJ and live drummer, the South Korean pop group wore lacy black mini-dresses and thigh-high leather boots, as if they were hosting a goth cocktail party. It was a rare American network television performance from a South Korean music group.

The song they performed on the January show, a slinky bit of minor-key dance-pop called "The Boys," owed an obvious debt to Kelis' catcalling hit "Milkshake." The band's gently lascivious choreography underscored the track's sex-appeal boasts: lead singer Kim Taeyeon made come-hither hand gestures while her bandmates pulled PG-13 versions of Lady Gaga's alien body bends. The song was in English, but the message was clear in any language. This was something new yet uncannily familiar on the American pop scene.

"As soon as I heard that we'd be performing there, I ran screaming and crying up and down our house," said Girls' Generation's Diamond Bar-raised, Korean American singer Tiffany. "The other members were just like, 'Huh?'"

Girls' Generation is arguably the biggest name in an effervescent, operatic Korean pop music culture that quietly has won a fervent fan base of young Korean Americans, and plenty of non-Koreans as well. K-pop artists pull from techno, hip-hop, R&B and top-40; singles are often focused vehicles for elaborate music videos and rarely less than bonkers good fun. Traditional Korean culture can be patriarchal, but K-pop's most famous acts, whose members often have roots in California, are groups of women deploying butt-kicking superhero imagery.

Poised at the intersection of two countries' fast-moving pop cultures and cutting-edge media technology, the sprawling genre colloquially known as K-pop has operated outside the American pop limelight. But that's changing. A-list producers like will.i.am, Diplo and Kanye West are lining up to work with South Korean artists like 2NE1, GD&TOP and JYJ.

K-pop comes alongside a tide of Korean filmmaking (the cult-favorite films of Joon-ho Bong) and culinary interest (L.A.'s Kogi truck, progressive Korean barbecue joints like LaOn Dining) turning heads in L.A. and in the U.S. As K-pop makes its first big moves into America this year with English-language tracks on U.S. major labels, a big question is this — does this music, at the vanguard of global pop, even need mainstream America at all?

For years, Korean pop lived in the shadow of Japan'shyper-kinetic music and fashion scene, whose anime culture stormed American television. But in 2009, one single instantly transformed the country's role in the Asian pop landscape. Girls' Generation's "Gee" was the K-pop equivalent of Elvis walking into Sun Studios: It drew the blueprint for a culture to come.

The song, written by the South Korean duo E-Tribe, used double-time electronic drums, fluorescent synthesizers and a cute-cloying repetition of the song's title. It's so insistent and poppy, it's almost avant-garde.

"It's just really good pop music. It's very hooky and fast and just doesn't sound like Western pop," said James Brooks of electronica band Elite Gymnastics, who wrote an essay on K-pop for the influential music website Pitchfork.

The track stormed Asia — the official version of the video where the nine girls dance around a clothing store clocking in at just over 70 million plays on YouTube. The song topped South Korean pop charts for two months and made Girls' Generation the first non-Japanese Asian girl group to top Japan's singles charts.

It also set a template that, alongside a broad array of peer acts like the more rap-inclined 2NE1 and dance-heavy group Wonder Girls, suggested that South Korea's pop music culture was coming into its own. Though the term "K-pop" is as reductive as referring to diverse U.S. artists like Lady Gaga, Skrillex and Lil Wayne as "A-pop," it captures the scope of the South Korean music celebrity scene.

The overwhelmingly single-gender bands, cast by talent agencies for Korean corporate label conglomerates like SM Entertainment and YG Entertainment, created fierce and ever-evolving loyalties — imagine picking your favorite Beatle or Rolling Stone if there were 10 of them.

Songs and especially videos were quickly passed over high-speed Internet and mobile devices several times faster than what's available in America. Sites like AllKPop and magazines like KoreAm chronicled the exploits of the young, fashionable and lightly transgressive stars — when GD&TOP and pop singer Hyuna saw singles banned by South Korea's major broadcasting networks, that made for delicious scandal. In August, Billboard launched a K-Pop Hot 100 chart to track the genre's sales.